Use extracted generic pubsub

[hannahhoward]

Goals

Use an extracted pubsub library that operates generically, so that fil-markets, graphsync, and others can avoid copying this pattern over and over

Implementation

• Take the pubsub code and make it mostly generic, but as type safe as possible
• Put it in its own library github.com/hannahhoward/go-pubsub

For discussion

Couple things going on here related to design that may not be obvious:

• library was extracted to my personal github cause I intend to use it not just in filecoin but also IPFS
  • it could live in IPFS but it's a simple enough and generic enough library I'm not sure it should be there either -- there is nothing IPFS-ish or Filecoin-ish about it.
  • I did do proper license attribution and put the protocol copyright in there
• there reason for the error return is not just for type checking but cause in graphsync we may want to short circuit transmitting to remaining callbacks in certain cases